Signal transduction via lymphocyte receptors dictates the fate of the responding cell: activation, deletion, anergy or ignorance. The functional outcome is immunity, tolerance or autoimmunity. CD45, a receptor-like protein tyrosine phosphatase, plays a critical role in regulating the function of Src familiy protein tyrosine kinases that play a key role in initiating and regulating signaling in multiple systems including by antigen receptors . Mutations in CD45 lead to immunodeficiency or autoimmunity in mice and in humans. Our lab previously found that dimerization of the CD45 cytoplasmic domain leads to the inhibition of CD45 functional activity. During the past funding cycle, we showed that CD45 spontaneously homodimerizes. Smaller CD45 isoforms dimerize more efficiently and are associated with lower activity. Dimerization inhibits CD45 PTP activity via a juxtamembrane "wedge"-like structure that interacts with and blocks the catalytic site of the partner monomer. A point mutation at the tip of the wedge eliminates the inhibitory effects of dimerization and results in a lymphoproliferative disease (LPD) and autoimmune phenotype in mice. During the last funding cycle, we studied the basis for the LPD and the autoimmunity in these mice and found that: a) the LPD and autoimmunity observed is subject to strain-specific genetic modifiers; b) involves multiple cell lineages; c) B cells are required for the LPD, are hyper-reactive and their development is altered; d) T cell development is affected and is associated with an increase in TCP responsiveness; and, e) the mutation leads to accelerated and more severe experimental autoimmune encephalomyelitis. These studies demonstrate CD45 function is negatively regulated by the juxtamembrane wedge. We hypothesize that the regulation of CD45 function by the wedge is critical in multiple cell lineages and that disruption of this regulatory function leads to autoimmunity via mechanisms that augment receptor signaling. In the next funding cycle, we propose to: 1) determine the influence of the CD45 E613R mutation on tolerance sensitivity of B and T cells; 2) determine the influence of the CD45 E613R mutation on myeloid cell functions; 3) understand how genetic modifiers of the CD45 E613R mutation contribute to autoimmunity; and, 4) define at a molecular level how the wedge mutation alters CD45 regulation and signaling.